This disclosure relates to patient-based detector crystal quality control for time of flight data acquisition. In particular, this disclosure relates to patient-based detector crystal quality control for time of flight data acquisition in positron emission tomography systems.
Positron emission tomography (PET) is an imaging method that is used in nuclear medicine and radiation therapy. During PET, a positron is emitted in a body due to radioactive decay. After a short distance, the positron enters into interaction with an electron. The interaction destroys both particles. The destruction creates a pair of gamma quanta. The quanta are at an angle of 180° from one another. The gamma quanta penetrate the body to be examined and after exiting it are recorded by two opposed detectors. A positron emission tomography scanner for imaging includes a plurality of gamma radiation detectors, which surround the patient to be examined.
The relevant radioactive decay may be induced, for example, by injection or inhalation of a radioactively marked radiopharmaceutical, such as a tracer. Disease information may be determined based on the spatial distribution of the tracer.
Radioactive decay involving the formation of positrons occurs during radiation therapy from the irradiation of a body, for example, as a function of the radiation dose applied. PET systems may perform such dosage validation or monitoring of the radiation therapy and particle therapy. In particle therapy, measurements are performed in order to check whether the planned radiation dose matches the dose actually applied and/or whether the spatial distribution of an applied dose matches a desired spatial distribution.
PET systems may be used with a particle therapy system and may deviate from the conventional ring form. For example, an in-beam PET system may include only two opposed detectors. The additional opening between the two detectors, for example, may be used to position the patient, or irradiate the patient with a beam passing through this opening without the beam striking the detectors.
To enable precise dosage validation, PET systems are calibrated at certain time intervals, for example, daily. Radioactive sources may be used for calibration. The radioactive sources are disposed in a treatment chamber in which the PET system is also located. The radioactive sources generate a defined activity, which is measured by the PET system. The measurements are used to calibrate the PET system. This process may, for example, include checking an existing calibration of the PET system.
PET scanner calibration is a routine procedure that is performed daily in order to provide accurate results when a patient is subjected to a scan. In some scanners, for example, data are acquired for about 20 to 30 minutes each day using a 20 centimeter (cm) diameter uniform cylinder. By assuming a known object (e.g., the 20 cm diameter uniform cylinder) an estimation of a crystal-efficiency normalization component is conducted, since the rest of the normalization components are fixed for a given scanner type.
Normalization factors are corrections that compensate for non-uniformity of PET detector pair efficiencies. A component-based method is used to improve accuracy of the normalization factors. Most components, such as geometric and crystal interference components, can be estimated in advance for a particular scanner type. This is contrary to the crystal efficiency component, which is estimated on a regular basis. Besides producing a normalization array, the crystal efficiency values are used in daily Quality Control (QC) procedures. In this procedure, particular block crystal sensitivities are checked against average block crystal sensitivities. A significant deviation of the block from an average one will signal for replacement or monitoring of this block. Potentially, data originating from this particular block can be excluded during list mode data histogramming and reconstruction.
The use of frequent phantom scans is not ideal. Self-normalization (estimation of the normalization array from unknown object data) was suggested as an alternative, but in non-TOF (time of flight), an acceptable solution can be achieved only with the use of significant a priori knowledge. The TOF self-normalization problem was proposed in, where crystal efficiencies were estimated with the help of detector singles measurements. However, such measurements are not available on all scanners. Similar information can be extracted from random events data on Siemens scanners. However, this singles estimation is of a low count nature and is used for random variance reduction. Singles modeling is equivalent to a non-collimated single-photon emission computed tomography (SPECT) problem formulation. This requires the development of an additional reconstruction model. Finally, singles efficiencies may not correlate well with efficiencies for coincidence events.
Calibration is therefore complicated, since dedicated radioactive sources have to be set up in the treatment chamber and then removed. This process requires manual intervention, involves cost, and can suffer from errors.